I've been following Apple and the rest of the industry since the mid 90's. Based on what I've seen I suspect that they already have built a handful of ARM Mac prototypes and maintain an ARM version of macOS, probably in a fairly serious way since the A7 shipped. macOS shares quite a bit of code with iOS and has been multi-architecture from the beginning. First bring-up was probably on actual iPhone or iPad dev hardware but they have the in-house fabrication capability to make machines that are visually the same as an existing model (probably 13" Air) with more or less the guts of an iPad. My guess is there are a small number of these floating around campus.

With those assumptions I still would bet against us seeing ARM Macs in the next couple years. There are a lot of advantages to keeping Macs single-architecture. Recent Intel CPUs are efficient enough that saving power there won't make a huge difference in user-available battery life (screen, DRAM, network, etc all use some too). Intel can price their CPUs aggressively enough to make cost not a major issue.

I do think it will eventually happen mainly because it will give them more control over their product line. Indicators of an upcoming shift will be if Intel screws up another generation of mobile CPU (it happened with Skylake, could happen again), Apple adds features like PCI-e for discrete GPU support (ok, they did in A9), and if Ax series performance starts to exceed corresponding Intel mobile parts by >50% (enough to compensate for the overhead of binary translation for legacy x86 apps).